Species concepts...

Before anyone can safely use algae for food, monitor blooms of toxic algae, use algae to detect changes in climate or water quality, or do any biological research involving algae, it is essential to have an adequate taxonomy - we have to be able to identify and name the organisms we are dealing with. One major problem is that we are still coming across many species of algae that have no name: discovering new species of algae is quite easy, even in well-studied areas like the United Kingdom.

But even where names do exist, there are often no good books or keys for identifying species. Worse, the species themselves are often poorly understood. When we use a name like 'Neidium ampliatum', which is a species of diatom, we do not know whether this name refers to a single species, like Homo sapiens (human beings), or Rhododendron ponticum (the purple rhododendron naturalized on acid soils in many parts of the UK); or whether it is equivalent to all the primates, or all monkeys, or the genus Rhododendron, or the family Ericaceae (to which Rhododendron belongs). So, we need to do some basic research, to find out what ‘species' means in algae.

Providing a basis for understanding

We have therefore decided to study a few carefully selected ‘model' taxa in depth, to provide a basis for understanding what a species is in microalgae and how species arise (the process of speciation). This will help establish a more rational basis for species taxonomy. We begin by studying the morphology of the organism by light and electron microscopy, and by using morphometric techniques (including computer-aided digital image analysis) to measure aspects of shape, size, and pattern. We also study the internal anatomy of cells and how cells develop, and the extent to which organisms change their appearance in response to different environments. We use molecular genetic methods to compare genes between different organisms and hence gain insights into evolutionary relationships between organisms. And we perform mating experiments, to determine whether different strains of organisms can interbreed (an application of the ‘biological species concept' - the idea that members of a single species share a single gene pool but are separated from other species by reproductive barriers).

Two major groups for study

We have selected two major groups of algae for study. The first group is the diatoms, the second largest group of plants (after the angiosperms) and major participants in the carbon and silicon cycles, accounting for approximately 20% of all C fixed per annum - more than all the world's tropical rainforests. They are increasingly used in environmental monitoring and palaeoecological reconstruction, in studies of climate and sea-level change, acidification and eutrophication, and some diatoms (Pseudo-nitzschia and Nitzschia species) produce the toxin domoic acid, which causes amnesic shellfish poisoning. The second group is the green algae, the third largest group of plants and especially important in freshwater and coastal marine ecosystems, where some (e.g. Cladophora, Enteromorpha) sometimes cause nuisance growths.

Studying the nature of species in diatoms

We have been studying the nature of species in diatoms for more than 25 years, after David Mann fortuitously discovered a new method to investigate mating processes in diatoms and found significant barriers to mating within two common freshwater species, Sellaphora pupula and Amphora ovalis. In the 1980s, we studied five diatom species in some detail in a small eutrophic lake in Edinburgh (Blackford Pond) and we looked at the relationship between morphological variation and mating patterns.

The most detailed work was done on Sellaphora pupula, where we showed the existence of six phenodemes (populations or sets of populations distinguishable phenotypically) growing together in the same pond. The phenodemes have maintained their identity for over 20 years, are subtly different in their morphology and size, and are differentially susceptible to parasitism by chytrid and oomycete fungi. They are unable to interbreed, as a result of prezygotic isolation mechanisms (i.e. the demes are unable to mate or, if they mate, the gametes are unable to produce a zygote) or, in one case, because the deme is asexual.

The microscopic structure of diatoms.

Since 1995, we have experimented on the demes in culture. We have isolated clones of several of the Blackford S. pupula demes, and clones of demes from elsewhere in the UK, Germany and the Ukraine, and grown them in unialgal culture. We have carried out the first crosses between isolates from geographically remote locations, e.g. crosses between British and Ukrainian clones, or between British and American clones. In the marine diatom Achnanthes longipes have demonstrated inbreeding depression during forced inbreeding in culture, in a deme that usually outbreeds. In three of the Blackford S. pupula demes too, we have found mechanisms promoting outbreeding, with a differentiation into ‘male', ‘female' clones and, in some cases, ‘hermaphrodite' clones.

We have also studied S. pupula demes using molecular sequence data, from 18S rDNA, ITS and rbcL sequences. The variation found among 18S and rbcL sequences supports the conclusions drawn from morphological and breeding data, that the demes are actually separate biological species.

More species than previously thought

Our overall conclusion, so far, is that many diatom ‘species', as they have been defined during the last 100-200 years, are composite and hide within themselves several to many entities that ought to be recognised as independent species. A 'species' like Sellaphora pupula is in fact equivalent to all the primates, or perhaps even all apes, and needs to be split into tens or hundreds of separate species, each of which probably has its own special physiology, ecology and geographical distribution. This leads us to the conclusion that there are probably c. 200,000 species of diatom on the planet, of which only about 10% have been recognised so far.
For more information on RBGE diatom research please visit our dedicate site: Algae World.

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